Particle–particle interactions during normal flow filtration: Model simulations

CFD-DEM study on transport and retention behaviors of nZVI-clay colloids in porous media

Transportation process of nano scale zero valent iron (nZVI) in clay-rich soils is complicated and crucial for in-situ remediation of contaminated sites. A coupled computational fluid dynamic and discrete element method (CFD-DEM) was used to investigate the interplays of repulsive and attractive forces and the injection velocity of this process. The screened Coulomb's law was used to represent the electrostatic interaction, and surface energy density was introduced to represent the effects of the van der Waals interaction. A phase diagram was constructed to describe the interplay between injection velocity and repulsive force (in terms of charge of colloids). Under the boundary and initial conditions in this study, clogging formed at low repulsive force (colloidal charge = -1 ×10-15 C), where increment of injection velocity (from 0.002 m/s to 0.02 m/s) cannot prevent clogging, as in the case of bare nZVI transportation with limited mobility; On the other hand, excessive repulsive force (charge = -4 ×10-14 C) is detrimental to nZVI-clay transportation due to repulsion from the concentrated colloids in pore throats, a phenomenon as in the overuse of stabilizers and was defined as the "membrane repulsion effect" in this study. At moderate charge (-1 ×10-14 C), injection velocity increment induced clogging due to aggregates formed at the windward of cylinder and accumulated at the pore throats.

Effect of long-range electrostatic repulsion on pore clogging during microfiltration

We perform computer simulations based on adhesive contact mechanics to demonstrate the clogging process of charged microparticles at the single-pore level. The effect of long-range Coulomb repulsion on clogging is characterized in terms of bulk permeability, the number of penetrating particles, and particle capture efficiency. Results indicate that the repulsion among particles delays or even totally prevents the formation of clogs. A clogging phase diagram, in the form of the driving pressure and a proposed charge parameter κ_{q}, is constructed to quantify the clogging-nonclogging transition. In addition, a critical state, where the capture efficiency of particles decreases to its minimum, is identified as a clogging-nonclogging criterion for repulsive particles. The distributions of the local volume fraction show that the structure of clogs is mainly determined by short-range adhesion. With relatively strong adhesion, a loose clog will be formed and it is easier for particles to penetrate. Finally, a schematic representation of the clogging process, considering both long-range repulsion and adhesion, is proposed to show the relationship between the clogging results and the interparticle interactions.

Colloidal Jamming Dynamics in Microchannel Bottlenecks

The purpose of this work is to examine the interplay between hydrodynamic conditions and physicochemical interactions from filtration experiments of microparticles. Experiments are performed in microfluidic filters with real-time visualization at pore scale. Both flow rate and pressure are measured with time to analyze the dynamics of pore clogging and permeability. Flux stepping experiments are performed at different physicochemical conditions to determine the different clogging conditions. The results allow distinguishing different clogging behaviors according to filtration conditions which are discussed by considering particle-particle and particle-wall colloidal interactions whose main characteristics are an important repulsive barrier at 0.01 mM, a significant secondary minimum at 10 mM, and low repulsive barrier at 100 mM. Clogging delay at moderate ionic strength and deposit fragility and associated sweeping out of aggregates of particles at high ionic strength are discussed from the deposit structure, specific resistance, and deposit relaxation analyses. It has also been observed that an opening angle at microchannel entrance causes rapid clogging, this effect being more pronounced when the repulsion is partially screened. Three different scenarios are discussed by analogy to crowd swarming: panic scenario (0.01 mM) where repulsion between particles induce pushing effects leading to the creation of robust arches at pore entrances; herding instinct scenario (10 mM) where the attraction (in secondary minima) between particles enhances the transport in pores and delays clogging; and sacrifice scenario (100 mM) where the capture efficiency is high but the aggregate formed at the wall is fragile.

Collective dynamics of flowing colloids during pore clogging

Based on direct numerical simulations of the coupled motion of particles and fluid, this study analyzes the collective hydrodynamic and colloidal effects of flowing microparticles during the formation of different 3D clogging patterns at a pore entrance. Simulations of flowing suspensions through a pore with various simulation conditions show that particle concentration and surface interactions play a major role in the occurrence of the bridging phenomenon (simultaneous adhesion of many particles). In the absence of DLVO repulsive forces, plugging is characterized by the temporal reduction of the bulk permeability when increasing the volume fraction of the flowing suspension up to 20%. Under these conditions, different structures of particle aggregates (from cluster to cake plug) are formed at the pore entrance yielding different evolution rates of hydrodynamic resistance to the flow. Taking into account DLVO repulsive forces in simulations for a particle concentration equal to 10%, we observe the transition from dendritic structures (for low repulsion) to dense aggregates (for high repulsion). At high DLVO repulsive forces, the scenario of pore clogging is controlled by the collective behavior of many interacting particles. It leads to the formation of a jamming phase (Wigner glass phase) with transient clusters of interacting particles at the pore entrance. The network of jammed particles collapses when the force chains among the particles are overcome by the flow stress. The build-up and the collapse of the jammed phase at the pore entrance induce temporal permeability fluctuations. According to the relative magnitude of particle-particle and particle-wall interactions, when the jammed phase is disorganized by the flow, the residual force in the network can accelerate particles and lead to particle adhesion at the wall inducing a pore blockage and a rapid reduction of the bulk permeability.

Colloidal surface interactions and membrane fouling: investigations at pore scale

In this paper, we examine the contributions of colloidal surface interaction in filtration processes. In a first part, we describe the way surface interactions affect the transport of colloidal particles or macromolecules towards a membrane, and its theoretical description. The concept of critical flux is introduced and linked to particle-membrane wall and particle-particle surface interactions. From this review, it seems important to consider how surface interactions occur at pore scale and control the development of fouling layers. In this context, we report in a second part experiments where the capture of micron-sized particles is observed in a poly-dimethylsiloxane (PDMS) microfluidic filtration device. Direct observations of the filtering part by video-microscopy allow to investigate the way the fouling of the microchannels by the particles is taking place. The experimental results underline the important role played by the particle-wall interactions on the way particles are captured during filtration. A small change in surface properties of the PDMS has important consequences in the way pore clogging occurs: in more hydrophobic conditions the particles first form arches at the microchannels entrance, then leading to the growth of a filtration cake, whereas in more hydrophilic conditions the particles are captured on the walls between the microchannels, then leading to the progressive formation of dendrites. To conclude, both experimental and theoretical approaches show the important role played by surface interactions in filtration processes. The complex interplay between multi-body surface interactions and hydrodynamics at nanometric scale leads to clogging phenomena observed experimentally in microfluidic systems that have not been predicted by numerical simulations. In the future, the two way coupling between simulation and experimental approaches at the pore scale have to progress in order to reach a full understanding of the contribution of colloid science in membrane processes.

Membrane processes in biotechnology: An overview

Membrane processes are increasingly reported for various applications in both upstream and downstream technology, such as the established ultrafiltration and microfiltration, and emerging processes as membrane bioreactors, membrane chromatography, and membrane contactors for the preparation of emulsions and particles. Membrane systems exploit the inherent properties of high selectivity, high surface-area-per-unit-volume, and their potential for controlling the level of contact and/or mixing between two phases. This review presents these various membrane processes by focusing more precisely on membrane materials, module design, operating parameters and the large range of possible applications.